Using an ice-penetrating radar to map the south pole's underlying terrain, the scientists calculated that the ice is up to 2.2 miles (3,500 meters) thick in places, said the study's leader, Jeffrey Plaut of NASA's Jet Propulsion Laboratory in Pasadena, California.

The radar, from the Mars Express orbiter, also revealed the surprising purity of the ice, Plaut added.

On average, the ice cap contained less than 10 percent dust, he said.
The study will appear in tomorrow's issue of the journal Science.

A Solid Find

The polar ice cap may also contain some frozen carbon dioxide, or dry ice, Plaut said. But there can't be much of it, because such a thick layer of dry ice would start to ooze sideways under its own weight. (Related photo: "Martian Geysers Spew Ice, Dust" [August 21, 2006].)

"Only water ice could support itself that way," Plaut said.

The research team also found a series of depressions buried beneath the ice only 180 miles (300 kilometers) from the pole.

These are probably impact craters, Plaut said, though they might also be features caused by erosion, similar to ones found elsewhere on Mars. (Related story: "Mars Has Liquid Water, New Photos Suggest" [December 6, 2006].)

"We don't completely understand them, because we have only a vague image of them," Plaut said.

But, significantly, the team didn't find a large depression under the ice.